I don't understand this. I know some people think that leftists are weak but that's not accurate according to studies. See for yourself!
"Among men of lower socioeconomic status (SES), strength predicted increased support for redistribution; among men of higher SES, strength predicted increased opposition to redistribution." (
https://journals.sagepub.com/stoken/rbtfl/aQgdXL1bx/R8k/full)
This means that poorer leftists tend to be stronger and only richer leftists are weaker generally speaking.
"According to those self-reports, people on both the right and left engaged in a greater amount of total physical activity than those in the middle — approximately one hour more each week." (
https://theweek.com/articles/441498/right-leftwingers-are-more-physically-active-than-centrists)
This one is self-explanatory.
Also, the stereotype of being a weakling is sometimes associated in the media with being a nerd, that is smart or educated and therefore also liberal. Obviously, such associations have some element of truth to them but they are massive oversimplifications of reality.